The present invention relates to dynamic braking of polyphase alternating current induction motors.
In many applications of alternating current (A.C.) induction motors, e.g., machine tool feed drives for milling, drilling and turning machines, it is desirable to rapidly stop motion of the motor when power to the motor is interrupted either intentionally or due to any designated fault. Various types of braking of A.C. induction motors are well known including mechanical friction braking, electrical braking using direct current injection into a stator winding of the motor, capacitive self-excitation and magnetic braking by short-circuiting the motor stator winding. Because mechanical apparatus to enable friction braking can become a significant portion of the cost of a motor, most braking of motors in the power range of from one to ten horsepower have utilized one or more forms of dynamic electrical braking. U.S. Pat. No. 3,153,182, for example, describes a braking arrangement for a polyphase A.C. induction motor which connects capacitors across each phase of the motor after the power source has been disconnected therefrom and, after the voltage generated by the self-excited motor falls to a predetermined level, connects braking resistors across the phases of the motor. U.S. Pat. No. 3,581,168 also describes an arrangement for connecting capacitors across each phase of an A.C. induction motor to initiate braking followed by short-circuiting of the phases after the voltage generated by the self-excited motor falls to a predetermined level. U.S. Pat. No. 4,311,948 describes an arrangement for connecting a capacitor across one phase of a polyphase A.C. induction motor and subsequently, after the self-excitation voltage has fallen to a predetermined level, short-circuiting one or more of the motor phases utilizing A.C. waveform phase control so that the short-circuiting only occurs during a portion of each A.C. cycle.
As exemplified by the above mentioned patents, the prior art has generally taught two-stage dynamic braking in which the second stage is initiated upon detection that the self-excitation voltage of the motor has fallen to a predetermined level. Accordingly, such systems have required some form of power supply coupled to the self-excited motor for producing electrical energy to maintain operation of a braking system while the motor is braked, an arrangement which cannot be used in a variable speed system since braking might be engaged when the motor is operated at low speeds and consequently at low voltage. In addition, the prior art systems have generally taught that the short-circuiting stage of braking be initiated such that the capacitors used in the first stage are short-circuited giving rise to potential welding of the short-circuiting contactors due to capacitor discharge currents.